In microfluidics, drops are usually confined between the floor and the roof of shallow microchannels. This causes drops to have a flattened shape. The description of droplet transport due to an external flow is different than in the case of spherical drops without confinement.
As a consequence, drops travel more slowly than the external carrier fluid, due to the friction between the channel walss and the drops. Besides, drops deform from the circular shape, always. In this project, we study these two effects.
We study the deformation of water drops immersed in oil subjected to a hyperbolic flow in confinemente. For that we designed a microchannel that produces a train of drops. Drops travel to a cross-shaped intersection where two opposed flows meet and exit in perpendicular directions. There, drops stretch due the flow. The faster the flow, the larger is the deformation of drops. Eventually, for fast enough flows, drops break up.
The microchannel is shallow and therefore drops have the shape of flat disks. This confinement, which we call δ, is key in the behavior of drops. We have found a law that predicts the deformation of drops taking into account the external flow, the drop size and the confinement. In particular, we have explained that it is the pressure field, and not the viscous stresses, the responsible for the deformation of drops. This explains the scaling that we observe in the deformation of drops as a function of the external flow and the drop size
A liquid column breaks up into drops due to interfacial tension. This is because any perturbation whose wavelength exceeds (approximately) the diameter of the jet is unstable. Although the drop size is mainly determined by the most unstable wavelength, the dispersion in the drop size reflects the instability of different wavelengths.
Our research focuses on the measurement of the dispersion relation of the capillaty jet. For that, we use Fourier analysis to find the spectral response of the jet. Since the most unstable mode dominates tha behavior of the jet, we have developed a mechanic focing technique to preferentially amplify different modes and thus study the behavior of the jet at different frequencies.
Droplet formation in microfluidics is very stable and robust. We want to use this fact to produce alginate microparticles of controlled size. Alginate is a biopolymer extracted from seaweeds with multiple current and potential applications. For example, alginate microparticles can be used to encapsulate cells or bacteria, since it is permeable to nutrients and gases and can act as a scaffold for the adhesion of cells.
Alginate crosslinks in the presence of multivalence cations, such as Ca+2. The main challenge for the production of alginate microparticles inside microchannels is to put in contact the alginate liquid drops with the solution of cations. This is because both are water soluble, while for the production of drops one needs immiscible fluids. Although it is possible to fuse alginate drops with drops containing the cations solution, it is difficult to sybchronize the drop production. For this reason, we are investigating the use of other substances that can dissolve the cations and, at the same time, be used to generate the alginate droplets.
Plasmons are waves that propagate at the surface of an array of scatterers, without radiating energy away from the array. Plasmons have been studied in the field of electromagnetism, but they are a generic wave phenomenon. Recently, we hace described the dispersion relation of plasmons when scatterers are sound-soft, that is, the wave field can penetrate into them. In particular, we have demonstrated that the impedance ratio between the external medium and the scatterers can tune the dispersion relation of the plasmons.
Our research now focuses on the experimental realization of acoustic plasmons in one- or two-dimensional arrays. For that, we use microfabrication techniques to build the array of scatterers.
Molecular diffusion is an ubiquitous process of great relevance at small scales. It can play a key role in the tranport of materia in porous media or inside cellular organelles. In such cases, the geometry can be complex and can determine the characteristic time scales of the diffusive process.
The description of diffusion inside a complex network of channels is not well described so far. In order to understand it we have fabricated networks of random channels with well defined statistical properties, such as the thickness and main length of channels. We fill these networks with a fluorescent solution and we study the diffusion of fluorescent molecules inside the networks. The objective is to understand the effect of the different length scales of the network of channels in the diffusion.